Synthetic analogues of [Fe ₄ S ₄ (Cys) ₃ (His)] in hydrogenases and [Fe ₄ S ₄ (Cys) ₄ ] in HiPIP derived from all-ferric [Fe ₄ S ₄ {N(SiMe ₃ ) ₂ } ₄ ]

<jats:p> The all-ferric [Fe <jats:sub>4</jats:sub> S <jats:sub>4</jats:sub> ] <jats:sup>4+</jats:sup> cluster [Fe <jats:sub>4</jats:sub> S <jats:sub>4</jats:sub> {N(SiMe <jats:sub>3</jats:sub> ) <jats:sub>2</jats:sub> } <jats:sub>4</jats:sub> ] <jats:bold>1</jats:bold> and its one-electron reduced form [ <jats:bold>1</jats:bold> ] <jats:sup>-</jats:sup> serve as convenient precursors for the synthesis of 3∶1-site differentiated [Fe <jats:sub>4</jats:sub> S <jats:sub>4</jats:sub> ] clusters and high-potential iron-sulfur protein (HiPIP) model clusters. The reaction of <jats:bold>1</jats:bold> with four equivalents (equiv) of the bulky thiol HSDmp (Dmp = 2,6-(mesityl) <jats:sub>2</jats:sub> C <jats:sub>6</jats:sub> H <jats:sub>3</jats:sub> , mesityl = 2,4,6-Me <jats:sub>3</jats:sub> C <jats:sub>6</jats:sub> H <jats:sub>2</jats:sub> ) followed by treatment with tetrahydrofuran (THF) resulted in the isolation of [Fe <jats:sub>4</jats:sub> S <jats:sub>4</jats:sub> (SDmp) <jats:sub>3</jats:sub> (THF) <jats:sub>3</jats:sub> ] <jats:bold>2</jats:bold> . Cluster <jats:bold>2</jats:bold> contains an octahedral iron atom with three THF ligands, and its Fe(S) <jats:sub>3</jats:sub> (O) <jats:sub>3</jats:sub> coordination environment is relevant to that in the active site of substrate-bound aconitase. An analogous reaction of [ <jats:bold>1</jats:bold> ] <jats:sup>-</jats:sup> with four equiv of HSDmp gave [Fe <jats:sub>4</jats:sub> S <jats:sub>4</jats:sub> (SDmp) <jats:sub>4</jats:sub> ] <jats:sup>-</jats:sup> <jats:bold>3</jats:bold> , which models the oxidized form of HiPIP. The THF ligands in <jats:bold>2</jats:bold> can be replaced by tetramethyl-imidazole (Me <jats:sub>4</jats:sub> Im) to give [Fe <jats:sub>4</jats:sub> S <jats:sub>4</jats:sub> (SDmp) <jats:sub>3</jats:sub> (Me <jats:sub>4</jats:sub> Im)] <jats:bold>4</jats:bold> modeling the [Fe <jats:sub>4</jats:sub> S <jats:sub>4</jats:sub> (Cys) <jats:sub>3</jats:sub> (His)] cluster in hydrogenases, and its one-electron reduced form [ <jats:bold>4</jats:bold> ] <jats:sup>-</jats:sup> was synthesized from the reaction of <jats:bold>3</jats:bold> with Me <jats:sub>4</jats:sub> Im. The reversible redox couple between <jats:bold>3</jats:bold> and [ <jats:bold>3</jats:bold> ] <jats:sup>-</jats:sup> was observed at <jats:italic>E</jats:italic> <jats:sub>1/2</jats:sub>  = -820 mV vs. Ag/Ag <jats:sup>+</jats:sup> , and the corresponding reversible couple for <jats:bold>4</jats:bold> and [ <jats:bold>4</jats:bold> ] <jats:sup>-</jats:sup> is positively shifted by +440 mV. The cyclic voltammogram of <jats:bold>3</jats:bold> also exhibited a reversible oxidation couple, which indicates generation of the all-ferric [Fe <jats:sub>4</jats:sub> S <jats:sub>4</jats:sub> ] <jats:sup>4+</jats:sup> cluster, [Fe <jats:sub>4</jats:sub> S <jats:sub>4</jats:sub> (SDmp) <jats:sub>4</jats:sub> ]. </jats:p>